1. Field of the Invention
The present invention relates to a brazing composition for aluminum materials, an aluminum material for brazing, and a method of brazing an aluminum material. More particularly, the present invention relates to brazing of aluminum materials comprising aluminum or aluminum alloys corresponding to brazing using an anticorrosive flux.
2. Description of the Background Art
Brazed products prepared by assembling aluminum materials or parts and brazing them have been widely used for heat exchangers such as automotive heat exchangers because of their lightweight and high conductivity. In the manufacture of such brazed products, various brazing methods are employed today, which are roughly divided into a flux brazing method and a vacuum brazing method. Among the brazing using a flux, brazing using an anticorrosive flux is in the mainstream, in which a compound containing fluorine (F), for example, fluorine compounds such as KF, AlF.sub.3, KAlF.sub.4, K.sub.2 AlF.sub.5, K.sub.3 AlF.sub.6, CsF, RbF, LiF, NaF, and CaF.sub.2, a mixture of these compounds, or fused and solidified products of these compounds is used as the main component.
In this brazing method, the flux is applied to a brazing material made of Al--Si alloys and heated to remove an oxide film on the surface of the aluminum materials to be joined. At the same time, the brazing material and the aluminum materials are provided with wettability to join the aluminum materials. As the brazing materials used in the above brazing using an anticorrosive flux, a brazing sheet comprising aluminum or aluminum alloys as a core material with brazing alloys such as Al--Si alloys clad on both sides or one side of the core material is used. Such a brazing sheet is manufactured by joining the core material and the brazing alloys by hot rolling and spreading to a predetermined thickness by cold rolling. This makes the manufacturing process complicated and increases the number of the manufacturing steps, thereby increasing the material costs. Moreover, since the brazing material contains hard Si, the molding cost also increases. Furthermore, in the manufacture of aluminum-made heat exchangers using such a brazing sheet, a step of applying an anticorrosive flux before heating for brazing further increases the cost.
In order to solve these problems of the prior art, a method which comprises applying a brazing composition comprising a mixture of metal Si powder and an anticorrosive flux to aluminum materials, assembling the aluminum material and the other material to be joined, and heating these aluminum materials to braze them without using a brazing sheet has been proposed (U.S. Pat. No. 5,100,048, U.S. Pat. No. 5,190,596, Publication of Translation of International Patent Application No. 504485/1994). This method uses a mixture of metal Si powder or other metal powders which form a eutectic alloy with aluminum, and a flux powder which removes an oxide film as the brazing composition. The mixture is applied to the aluminum materials to be joined and heated while the metal Si powder or other metal powders which form a eutectic alloy with aluminum in the mixture is in contact with the aluminum surfaces to diffuse Si and the like in the aluminum material and create a state similar to a eutectic structure of eutectic alloys such as Al--Si alloys with the aluminum components to join the aluminum materials.
However, in this method, the metal powder which forms a eutectic alloy with aluminum, such as a metal Si powder used as a component of the mixture, is relatively expensive. Moreover, the powder grinding operation for preparing the metal powder to a predetermined particle size is difficult because of its hardness, thereby increasing the manufacturing cost.
If a small amount of the metal powder (Si) having a large particle size is included in the mixture, such a metal powder fuses the aluminum material that is a base metal during the steps from heating to cooling for brazing to cause a large amount of local fusion. This brings about problems in the resulting brazed products. For example, in the case of heat exchangers, the necessary characteristics such as pressure resistance and corrosion resistance may be impaired. In the case of brazing parts which require a large amount of a brazing composition, if the mixture of the brazing composition is applied thickly, the metal (Si) does not react with the aluminum components in the aluminum material and remains as a black residue.